1. Field of the Invention
The present invention relates to a multistep electronic circuit device comprising electronic circuit elements, wiring substrates and cooling elements, more particularly to an electronic circuit device capable of being easily assembled in spite of the complicated order of assembling the device, easily disassembled for maintenance, and highly reliable for a long period of time.
2. Description of Related Art
It has hitherto been proposed to assemble various electronic circuit devices such as a computer and electronic automatic exchange with various kinds of solders for many purposes of giving high-speed operation and many functions to the devices and miniaturizing the devices. Particularly, typical examples of prior art high-density electronic circuit devices are one which was assembled with up to three solders, as shown in FIG. 2 attached hereto (see Nikkei Electronics, Mar. 26, 1984, p. 161) and one as shown in FIG. 17 attached hereto (see Japanese Patent KOKAI (Laid-Open) No. Sho 60-226142). In FIG. 2, input and output pins 5 are bonded to a substrate 4 with an AuSn system solder. In FIG. 17, input and output pins 5 are bonded to a circuit board 3 with a Ag solder 4, then LSI chips 6 are bonded to the substrate 3 with a solder 7 of Pb5wt%Sn and a sealing cap 1 is bonded to the substrate 3 with a solder 2 of Sn37wt%Pb eutectic alloy. These electronic circuit devices are featured by being made with the maximum three kinds of solders in such a manner that LSI chips are bonded directly to a substrate to realize the high density of the device (referred to as a multi-chip mounting method) or through the basic steps of connecting input and output pins to a substrate, bonding LSI chips to the substrate and air-tight sealing the device with known solders by melting the solders. Solders used for each of the bondings are Ag solders and AuSn solders for the bonding of the pins, a Pb5wt%Sn solder for the bonding of the LSI chips, and a Sn37wt%Pb solder for the sealing.
However, the electronic circuit devices required to be further improved in their performance were hardly realized by prior art structures of device and prior art solders.
For the purposes of improving the performance of the electronic circuit devices, i.e., giving high-speed operation and many functions to the devices, miniaturizing the devices and making the devices highly reliable, fundamentally (1) LSI chips must be integrated in a larger scale and (2) such LSI chips must be mounted in a higher density. That is, more logic circuits and memory circuits are built in one chip and these LSI chips are assembled in a high density, and in addition, the leads between the chips are made as short as possible, thereby improving a transfer speed of signals.
However, the above-mentioned manners are accompanied with the following difficulties: when the LSI chips are integrated in an ultra-large scale with increased amounts of logic circuits and memory circuits, (1) that a great number of signal-input and output terminals and power terminals for driving the LSI chips are required, (2) that high-speed switching requires ultra-high frequency circuits of several hundreds of MHz to several hundreds of GHz, (3) that this high-speed switching of the great number of circuits requires an increased amount of power, so that the generated amount of heat is highly increased, and (4) that formation of the circuits in a micron or submicron order causes the LSI chips to be very sensitive to surface contaminations due to various impurity ions and/or various gases in each of the steps of assembling the LSI chips. The highly reliable, miniaturized and easy-assembled LSI chips are obtained by overcoming the above-mentioned difficulties.
There are also some difficulties in making high-density mounted, finely patterned LSI chips with improved transfer speed of signals on a substrate and with very short leads, and in using a ceramic having a low dielectric constant and an organic thin film circuit.
Prior art techniques cannot overcome all these difficulties and avoid the contamination of the LSI chips. Furthermore, there is also such a problem that the heat generation of the LSI chips prevents the cooling of the device with good efficiency.